Abstract
Arbuscular mycorrhizal fungi associate with roots of the majority of land plants and supply up to 80% and 25% of their P and N requirements, respectively. These fungi do not form mycorrhizas on individual plants in isolation, the way they often have been investigated. Instead, arbuscular mycorrhizas may form when hyphae connected to one root system branch throughout the soil while foraging for mineral nutrients, encounter, and colonize the roots of another plant, thereby forming a common mycorrhizal network (CMN). The importance of CMNs is their potential influence on the distribution of limiting mineral nutrients among plants. Although it is likely that most research on plant interactions may have incorporated CMNs unwittingly, until recently, few investigators have attempted to distinguish the effects of CMNs. Throughout my dissertation research I have found that CMNs mediate belowground plant interactions. In Chapter One, I introduce arbuscular mycorrhizal fungi, their relationship with host plants, and the processes involved in the formation of CMNs. In Chapter Two, I investigate whether effects of CMNs can be detected on populations of Andropogon gerardii, a dominant grass species of tallgrass prairies. I found that CMNs intensify intraspecific competition among A. gerardii, and the competitive effects of CMNs increase size inequality within populations. In Chapter Three, I experimentally test the mechanisms behind the intensified competition found in Chapter Two. I learned that CMNs amplify competition through reciprocal rewards between large, abundantly carbon-fixing plants and AM fungi, thereby suppressing the growth of small, neighbor plants. Chapter Three reaffirmed the results of Chapter Two by again finding that CMNs intensified competition and increased size inequality among plants with intact CMNs versus those with severed CMNs. Chapter Four includes a second plant species by contextualizing the dependence upon mycorrhizas and responsiveness to mycorrhizas of A. gerardii and Elymus canadensis, a prairie sub-dominant grass, in preparation for the research of Chapter Five. I also consider how these mycorrhiza-related attributes influence plant functional traits. I found that A. gerardii is more dependent on AM fungi for mineral nutrient uptake and growth than is E. canadensis, although when fertilized, both species can grow without AM fungi. Then, using a phosphorus amount that I had determined to favor strong mycorrhiza responses by both species, in Chapter Five I went on to investigate how these two contrasting tallgrass prairie plant species interact via CMNs. I discovered that intact CMNs increased survival and growth of both species in monocultures and in mixture. For E. canadensis, intact CMNs improved plant water uptake, likely by increasing access beyond cone-tainers. For A. gerardii, intraspecific interactions were more intense than interspecific interactions with E. canadensis, and intact CMNs resulted in substantial overyielding by A. gerardii when in mixture with E. canadensis. In Chapter Six, I synthesize the four data chapters and suggest that it is not just the presence of AM fungi that has implications for grasslands, but it is the interconnecting hyphae of CMNs that mediate plant interactions. Notably, my work found that CMNs improved Mn acquisition, enhanced the mycorrhizal colonization of putatively carbon-limited plants, and increased stomatal conductance of E. canadensis. The extent to which such effects of CMNs can be realized, however, likely depends upon conditions of soil fertility, plant density, and whether or not root systems overlap. Hence in nature, CMNs may be a “mixed blessing,” either benefiting plants through improved mineral nutrition and hydration that enhance survival and growth, or disadvantaging them by intensifying competition especially to the detriment of small individuals.